X-ray tubes typically consist of a cathode assembly opposing an anode assembly contained within a vacuum tube. The basic cathode assembly is a filament that is recessed in a cup-shaped structure. When energized by a filament power supply, the cathode's filament heats up to extremely high temperatures and electrons are boiled off. The anode, which is typically tungsten, is located at the opposite side of the X-ray tube and is oppositely charged from the cathode. The positively charged anode attracts the negatively charged electrons expelled from the cathode. The electrons are accelerated towards the anode at great speed and collide with the anode with great force. The interaction between the colliding electrons and the tungsten atoms in the anode create high energy X-ray photons which can be used to perform noninvasive internal examinations because of their ability to pass through objects.
The X-ray radiation is produced in a small area on the surface of the anode called the focal spot. The size of the focal spot is determined by the size of the electron beam at the anode and is an important characteristic of the X-ray tube. The size of the focal spot essentially determines the resolution that can be obtained with any given X-ray tube. Small focal spot sizes produce less image blurring and is critical for higher resolution images in devices such as CT scanners. While small focal spot sizes provide for greater resolution, they also produce more heat in the assembly because the electron beam is concentrated in a small area on the anode. This high heat can damage the device unless mitigation techniques are utilized to reduce the heat before damage to the anode occurs.
Oversampling is a technique that is used to obtain higher resolutions in CT scanners using digital detectors while reducing anode heating. To achieve oversampling, the focal spot is moved between two successive views on the anode using electrostatic means. This is accomplished by arranging several electrodes in close proximity to the electron beam. The electrodes are energized to shape and deflect the electron beam as the beam leaves the cathode.
For good focal spot quality and repeatable results, the beam shaping and deflecting electrodes need to be manufactured to extremely tight tolerances and placed at consistent locations with respect to the electron beam. Changes in either the location of the electrodes or the dimensions of the electrodes requires individual X-ray tube testing, calibration and focal spot control adjustment for each X-ray system produced which is time consuming, costly and prevent drop-in replacement should the X-ray tube need replacing. At a system level, electrode placement must be consistent else each system would require specific calibration to compensate for variance in electrode placement.
Traditionally, the manufacturing method for producing multi-electrode cathode assemblies for X-ray tubes involved first machining the electrodes and then subsequent assembly of a complex cathode structure. Each individual electrode would be separately positioned in the cathode assembly and then bonded into place. This method of manufacture requires precise machining of a plurality of electrodes and then high accuracy in placing and bonding the several electrodes in their required location. The manufacture of such a multi-electrode system presents a formidable challenge because of the difficulty in placing and bonding the finished electrodes to tight manufacturing tolerances.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a method of producing a multi-electrode cathode assembly wherein the beam shaping and deflecting electrodes can be accurately and repeatedly located with respect to the each other and to the cathode filament, in a less costly and less time consuming manner than the traditional method There is also a need for improved a multi-electrode cathode assembly which has beam shaping and deflecting electrodes located at precise and repeatable locations and which is also is less difficult to manufacture.